The returnenvelope keyword takes a single integer value, which is interpreted as a set of bit flags. Bit 0 (value = 1) controls whether or not return notifications generated by the MTA are written with a blank envelope address or with the address of the local postmaster. Setting the bit forces the use of the local postmaster address; clearing the bit forces the use of a blank address.
The use of a blank address is mandated by RFC 1123. However, some systems do not properly handle blank envelope From: addresses and may require the use of this option.
Bit 1 (value = 2) controls whether or not the MTA replaces all blank envelope addresses with the address of the local postmaster. This is used to accommodate noncompliant systems that do not conform to RFC 821, RFC 822, or RFC 1123.
Bit 2 (value = 4) prohibits syntactically invalid return addresses.
Bit 3 (value = 8) same as mailfromdnsverify keyword.